Hot melt adhesives, or adhesives that are solid at room temperature and which must be melted prior to use, are used in an increasing number of applications. For example, hot melt adhesives may be used for coating substrates, for sealing of packages, for building construction, shoe manufacturing, bookbinding, for the assembly of automobile parts, electronics, electrical equipment, appliances, electrical components, furniture, and for metal-to-metal bonds, to name but a few. Because the hot melt adhesives are solid at room temperature, it is necessary to melt the adhesive prior to application.
A common type of hot melt adhesive is the type known as a moisture cure adhesive. As the name implies, moisture cure adhesives cure in the presence of water molecules in the ambient air. This aspect of moisture cure adhesives poses a problem for systems that melt and dispense the material. Typically, systems for melting and dispensing moisture cured hot melt adhesives include a lidded hopper for holding the solid hot melt adhesive and means for melting the adhesive, such as a heated grid at the bottom of the hopper. During use, the system is sealed from the ambient air to prevent premature curing of the adhesive. However, when adhesive is added to the system, ambient air having sufficient moisture to cure the hot melt adhesive may also enter the system. To reduce the premature curing of the moisture cure adhesive, dry air or an inert gas may be injected into the hopper of the hot melt adhesive system to prevent any moisture laden ambient air from contacting the adhesive.
With existing devices, the inert gas or dry air is typically injected into the hopper by a single jet in a horizontal plane and perpendicular to the wall of the hopper. Although this system may aid in preventing ambient air from being forced down onto the molten hot melt adhesive already within the hopper, it may also serve to trap moisture laden ambient air between the surface of the molten hot melt adhesive and the plane of the injected gas. Thus, existing systems may exacerbate the problem of premature curing. Moreover, existing systems also may suffer from additional drawbacks.
First, polyurethane reactive adhesives or PUR, a common type of moisture-cured adhesive, typically uses methylene bisphenal diisocyanate (MDI) as a curative. The vapors generated by the hot melt will contain particles of this curative. OSHA dictates that the MDI levels in the operator's environment not exceed 5 parts per billion. Thus, many adhesive manufacturer's recommend that adequate ventilation be provided. However, existing systems for melting and dispensing moisture-cured hot melt adhesives often rely on exhaust systems external to the hot melt dispensing device rather than integral therewith. In these types of devices, after adhesive is added to the hopper, the lid of the hot melt adhesive apparatus is closed. As inert gas or dry air is injected into the hopper, the injected gas creates a positive pressure therein. Because of the positive pressure, the inert gas or dry air, as well as vapors from the hot melt adhesive, may be forced through internal leak paths, such as at joints and fittings, and out into the operator's environment, thereby evading the external exhaust system.
Alternatively, existing systems that do include an integral exhaust system within the hot melt dispensing device suffer from the contrary problem. When the lid of the hopper is in an open position, the exhaust system is free to draw not only the vapors from the hopper, but also ambient air from outside of the system. However, when the lid is closed, the exhaust system continues to draw gas from the hopper, creating a negative pressure within the hopper because the airflow of the exhaust system typically exceeds that of the airflow of the inert gas or dry air into the hopper. This negative pressure may cause moisture laden ambient air to enter from the environment into the hopper through any leak paths in the unit. This entry of moisture laden ambient air may cause premature curing of the adhesive within the hot melt dispensing device.
Therefore, there is a need for an adhesive curing abatement system that is effective in reducing the premature curing of hot melt adhesive within a hot melt dispensing device without the foregoing disadvantages of existing systems. Further, there is a need for a moisture cure abatement system that includes an integral exhaust system which eliminates the positive pressure within the hopper of the hot melt dispensing device without drawing moisture laden ambient air into the hot melt adhesive system.